I don't know OpenGL or Slick, but it seems like the x/y are the origin to rotate around, and so when you set it as (0, 0) you are rotating around bottom left. To calculate the origin (you probably already know) just take the image x + (imageWidth / 2), and image y + (imageHeight / 2).

Alright, so the angle and the z-coords are the only ones that need editing considering I am doing a 2D game, so how would I stop it from rotating in a fixed radius around the origin, and have it stay on the origin whilst rotating, or being able to set the radius it rotates around the origin?

EDIT: That tutorial told me how to translate the position properly, so all is good

java-gaming.org is not responsible for the content posted by its members, including references to external websites,
and other references that may or may not have a relation with our primarily
gaming and game production oriented community.
inquiries and complaints can be sent via email to the info‑account of the
company managing the website of java‑gaming.org